This invention relates to a fluidized bed boiler and to a method for the rapid ignition of the combustible fluidized bed.
A fluidized bed is a mass of discrete particles suspended in an enclosure by a flowing fluid stream. The fluid, which is commonly a gas, enters the enclosure through a porous bottom surface and passes upward through the particles. Individual particles are separated from one another and suspended in the stream so that the mass of fluidized particles resembles a high viscosity liquid. An overview of the theory and practice of fluidization is given in Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd Edition, Volume 9 (1966), pp. 398-445.
Recently developed fluidized bed boilers employ oxidizing fluidized beds for heat generation. A bed of inert granular material within the boiler is suspended in contact with heat exchange surfaces by a stream of air. When the fluidized bed is heated above a critical ignition temperature and supplied with a mixture of air and coal, intense combustion occurs. High heat releases, and heat transfer direct from the bed material to the heat exchange surfaces enhance the efficiency of the boiler, reducing the dimensions required for a specified thermal output, in comparison with more conventional boiler designs.
When a fluidized bed boiler is activated, the mass of bed material must be heated above the fuel ignition temperature. For large beds, the mass of material involved makes simultaneous startup of the entire bed expensive and inefficient. Coal is difficult to burn in a fluidized bed until the bed is raised to a relatively high temperature. Using external burners to heat a bed to the coal burning temperature, an hour or more may be required. Fluidized beds, without heat exchangers installed, have been heated to combustion temperatures by firing with external burners from below or from above the bed. Depending on the size of the reactor, this may take several hours. Experimental fluidized bed boilers have been heated the same way. In small experimental fluidized bed boilers, a gaseous fuel has been burned within the bed for a rapid heating. However, there are no safe means to do this in a large fluidized bed boiler.
It has been proposed to start a bed in increments, using an already active portion to ignite an adjacent portion. One such method employs a bed with a divided air supply. Fluidizing air is first supplied to a small section of the bed, and after it ignites increasingly larger areas are fluidized, each area activating in turn by the effects of particle and fuel migration. A second method, described in U.S. Pat. No. 3,387,590, employs controllable ports in the partitions of a divided bed. After one area of the bed is activated, the ports leading to another area are opened so that bed material and fuel migrate to the inactive area. A modification of this second method, using continuously open ports in association with the normal operating characteristics of the bed to eliminate the need for mechanical controls is shown in U.S. Pat. No. 3,736,908. U.S. Pat. No. 3,769,922 shows preheating a bed by passing a stream of preheated gas downwardly therethrough until it is sufficiently heated and then fluidizing the bed with an upwardly flow gas stream.